When a film of an organic thermoplastic, crystallisable polyolefin, such as polypropylene, is drawn to cause molecular orientation therein and thereby improve its physical properties, the resulting film will normally have, for many applications, an undesirably high shrinkage at elevated temperatures. Proposals have been made for improving the dimensional stability at elevated temperatures of such oriented films by processes known in the art as heat-setting operations in which an oriented film is heated, while restrained against shrinkage, at a temperature above the glass transition temperature (Tg) of the polymer and below its melting point. The optimum heat-setting temperature can readily be established by simple experimentation, and in practice heat-setting of a polypropylene film is usually effected at temperatures in the range of from 100.degree. C. to 140.degree. C. Heat-setting may be effected by conventional techniques--for example by means of a stenter system, or by a system of one or more heated rollers as disclosed, for example, in British Pat. No. 1,124,886. In a conventional heat-setting operation of this kind a biaxially oriented film, although restrained against shrinkage, is, in practice, permitted to shrink in controlled fashion by a significant amount. For example, an area shrinkage (the sum of the linear shrinkages in each of the longitudinal and transverse directions) of the order of 20 to 25% may be tolerated.
We have now devised an improved heat-treating technique for oriented polyolefin film.